A jack-up rig is an offshore oil exploration drilling structure for use in shallow water, typically in water depths up to 500 feet. The jack-up rig normally comprises a floatable hull with a deck or working platform and three or four legs. After arriving on location, the legs of the jack-up rig are lowered until they touch a seabed beneath the hull. This allows the hull to be supported by the legs that rest on a foundation soil on the seabed so that the hull may be jacked up using a jacking system to raise the working platform above the water, making the jack-up rig safer to be operated in open water situations where water movement is experienced.
The legs of a jack-up rig are commonly trusses, each truss comprising vertical chords connected with cross braces that are normally diagonally disposed. The legs normally terminate in a footing that rests on the seabed. The footing provides an enlarged soil bearing area so as to reduce pressure exerted on the soil of the seabed. This in turn reduces bearing capacity that is required by the soil to support the jack-up rig, allowing the jack-up rig to be operated in a greater variety of locations and soil types.
Spud can footings are individual footings each connected to one leg of the jack-up rig. This allows the jack-up rig to be used on uneven seabeds and on slopes as the length of each leg may be independently adjusted relative to the other legs. A spud can is typically shaped like a top, having a generally conical upper half connected to the leg and a generally conical lower half for contact with the seabed. The conical bottom half helps ensure some penetration into the seabed, even in very stiff soils, so as to provide some anchoring of the legs into the seabed.
To reliably support the hull above water, the legs of the jack-up rig must be installed on the seabed in a way that can withstand maximum expected loads, such as those experienced in extreme storm conditions or during drilling operations. To assure sufficient bearing capacity of the seabed, an operation known as preloading must be performed for jack-up rigs by ballasting the hull with sea water. In the preloading operation, soil at the seabed supporting the legs is artificially loaded to a full load that can be expected when the jack-up rig is in its extreme or most severe condition, normally at “storm survival mode”. This is to reduce the likelihood of soil failure during the extreme condition, leading to catastrophic consequences for the jack-up rig.
During preloading, the legs of the jack-up rig are allowed to penetrate the seabed under the weight of the jack-up with its hull ballasted, with the spud cans failing the soil of the seabed until a point where the seabed resistance finally equals the load imposed by the spud cans. When there is no further settling of the legs into the soil for a predetermined holding period, preloading is complete and the hull can be safely jacked up to full operational air gap above the water.
During preloading, there is a risk of soil failure of the seabed under the spud can footings. Some modes of soil failure include punch-through and rapid leg penetration. Punch-through is an extreme event and may occur in a seabed composed of strong overlying weak layers, for example, a sand layer overlying soft clay. When applied load during preloading exceeds the bearing capacity of the sand layer, the sand layer will suddenly give way and the spud can will punch-through the sand layer to plunge into the underlying soft clay. The leg experiencing this failure will continue to penetrate the seabed until either the soil is once again able to support the leg, or the hull enters the water to a point where buoyancy of the hull provides enough support of the entire rig to halt further leg penetration into the soil. Rapid leg penetration may occur if preloading of a leg takes place on unexpectedly soft or weak soil and the leg moves downward during preloading faster than the jacking system is able to actively jack up the hull.
When punch-through or rapid penetration occurs, the hull becomes out of level, causing all the legs to experience increased transverse loads. Some of the load previously carried by the leg experiencing the soil failure is also transferred to the other legs. Accidental loading resulting from a punch-through or rapid leg penetration can lead to several types of leg damage including buckling of the braces, buckling and/or shearing of the chords, joint damage and even damage to the jacking system. Such damage is extremely costly and rig operators have to be insured to cover the risks.
To minimize the occurrence of punch-through or rapid leg penetration, proper testing of the foundation soil where a jack-up rig is to be sited is extremely important, so as to avoid attempting to install a jack-up rig on soil that is unable to appropriately support the rig with an adequate safety margin.
Soil testing for jack-up rigs is normally carried out by conducting one or more geotechnical surveys prior to towing the jack-up rig to a drilling location. Such a survey typically includes borehole tests with various shear strength tests conducted on site and/or in a laboratory, as well as piezo-cone penetration tests (PCPT) at various points about the intended rig location. Results from the geotechnical surveys allow spud can penetration predictions to be made, as well as other geotechnical aspects to be evaluated.
Although geotechnical surveys may be conducted for a site, it is often found that information obtained in such surveys is deemed insufficient or inaccurate upon actual going on location and installing the jack-up rig. This may be due to an offset of boring locations during the geotechnical survey from the final installation site, with the problem being exacerbated in areas where there is considerable lateral variation of the seabed and where soil testing has only been undertaken at a limited number of points about the site. Another cause for discrepancy may be associated with re-orientation or adjustment of the rig location during actual installation, resulting in a deviation of the final location from the originally intended and surveyed site.
If geotechnical information is insufficient or there is doubt as to the reliability of existing information and associated spud can penetration prediction, further soil testing are necessary and may be carried out using a drilling cantilever extending from the rig itself. In such cases, the jack-up rig is first towed to location and allowed to come to rest on the seabed. Soil testing using the drilling cantilever is then carried out prior to conducting the preloading operation. However, there are inherent risks to the stability of the structure when extending the drilling cantilever for operations before the jack-up rig has been properly preloaded and installed on site. In addition, conducting such soil testing using the drilling cantilever will naturally delay installation of the jack-up rig for some time.